The present invention concerns a burner arrangement for the combustion of a fuel gas/oxygen mixture, especially for use for the preparation of fuel gas in a fuel cell system.
Fuel cell systems require hydrogen as the energy source. This hydrogen is frequently produced by an endothermic conversion process from liquid energy carriers with a high H/C ratio. The necessary process heat is supplied by exothermic reactions which are implemented via autothermic or allothermic process modes. The combination of heat generation and hydrogen production units forms the fuel gas preparation system (a.k.a. xe2x80x9cfuel processorxe2x80x9d) for the actual fuel cells, which are frequently arranged spatially separated from the fuel gas preparation system.
In the autothermic reaction mode, the heat is generated and consumed directly inside a reaction zone, i.e. at least one exothermic and one endothermic reaction are linked together. However, this leads to poor quality of the product stream.
In the allothermic process mode the heat source, i.e., the fuel arrangement and the heat sink, e.g. the reformer unit, are geometrically separated, but may be arranged directly side by side. This means that the reformer unit is supplied from the outside with heat, said heat being implemented by a combination of convection, conduction and radiation. In other words, the exothermic reaction in the burner arrangement and the endothermic reaction in the reformer unit are locally separated from each other.
Predominantly, catalytic burners are used in the allothernic reactor design for fuel cell systems. During catalytic combustion, the gaseous fuel gases react with oxygen (usually supplied in the form of air) on the surface of a fixed catalyst. Typical catalysts are the noble metals, platinum and palladium. The quantity of energy required for starting the reactionxe2x80x94the activation energyxe2x80x94is reduced by the catalyst. As a result, the conduction temperatures are clearly reduced and the thermal NOx, formation is avoided. An almost complete reduction of the educts is assured by the catalyst. In addition, the catalytic burners operate with very little pollution.
Because of the above mentioned advantages, catalytic burners are generally used for fuel preparation systems. However, a number of disadvantages exist such as:
it is difficult to make the heat coupling efficient; it is usually accomplished by conduction and convection via a carried medium,
energy losses arise due to the spatial separation from the heat sink,
the arrangement tends to favor a large structural volume and has a corresponding high weight,
lag times arise in the operating magnitudes in the case of dynamic load variation.
The object of the present invention is to devise a burner arrangement which leads to an efficient coupling of heat, avoids energy losses due to the spatial separation of the heat sink which has a relatively small weight and can therefore be operated in such a way that the heat generation can be better adapted to the dynamic load variation conditions with relatively short lag times.
To solve this problem, the present invention envisions a burner arrangement for the combustion of a fuel gas/oxygen or air mixture characterized by a body which is permeable for the mixture whose surface areas defining the free cross section of flow are covered with an oxidation catalyst, by a feeder device for the mixture arranged on an inlet side of the permeable body which distributes the mixture over at least essentially the entire inlet area of the inlet side and by a layer coordinated with the feeder device separating the catalytic combustion zone of the permeable body from the inflow of mixture, but permeable for it, which serves as a flashback safety.
The permeable body is preferably rectangular when viewed from the top, especially square, and can therefore be inserted in a sandwich-like structure with alternating zones of heat generation and heat consumption, despite the fact that the permeable body has flow passing through it perpendicular to its flat area and not along its flat area as is the case, for example, in the fuel preparing system known from EP-A 0 861 802.
In this fuel preparation system, the zones for catalytic combustion, reforming, evaporation, superheating, etc. alternate with each other, in which case a strongly endothermic reaction stage such as reforming must necessarily be surrounded on both sides by heat-supplying combustion stages. The catalytic combustion conventionally takes place there on pellets, which are spatially fixed in a flat zone, in which case the mixture flows along the flat zone. The above mentioned problems are also true here. It has been found that arrangements with catalytic pellets are disadvantageous for various reasons.
The burner arrangement according to the present invention also necessitates a certain structural height, but has the fundamental advantage that when the burner arrangement is put into operation and possibly when it is in operation, not only is catalytic combustion, but normal combustion is also possible, on or directly above the outlet side of the permeable body. As a result, the generated heat, on the one hand, causes a rapid heating up of the permeable body coated with catalyst without the need of charging the burner arrangement with preheated gases. On the other hand, heat is efficiently transferred to the neighboring endothermic stage or stages both during operational startup of the burner arrangement with the aid of normal combustion and also in the case of catalytic combustion in the permeable body after startup. The heat is balanced out primarily by radiation to the opposite surfaces in each case. This is highly efficient, because according to the Stefen-Boltzmann law, the temperatures enter in with the fourth power. Convection and conduction also take place. Their involvement in the heat transfer, however, is clearly smaller.
In steady state operation of the fuel preparation system it is easily and simply controlled, because the surface temperature of the burner arrangement can be adjusted precisely via the enthalpy stream of the incoming fuel gas. The heat flux into the reforming zone or reforming zones can be governed by radiant heat exchange. Since the burner operates almost completely catalytically, the pollutant emissions are very low.
The energy from the hot burner exhaust gases can be cycled back into the process economically via a heat recovery device, such as a heat exchanger, and is therefore not lost.
In the case of dynamic operation o the fuel preparation system, e.g., in the case of mobile use in a car, very strict load change requirements exist with respect to the hydrogen demand in the burner arrangement. The fuel preparation system must always be capable of covering this hydrogen demand. The time constants for variations of the hydrogen flow lie in the millisecond range so that different requirements on the catalytic burner arrangement result. In other words, it must be capable of realizing fast load cycles for the streams of educt material, of achieving complete reaction of the educts within this time constant and assure an efficient and sufficiently fast heat transfer. The management of the material flows, i.e. the sluicing in and out and the regulating of the material flows presents no system-engineering obstacle. The modulation of a purely catalytic operating burner, conversely, cannot be realized within the above known time constants. The complete reaction of varying streams of educt lies in the second range. A conventional purely catalytic burner reacts too slowly in the case of load change jumps. Complete combustion cannot take place. However, it is possible by using the invention, via the also reliable homogeneous flame combustion, to design the burner arrangement in such a way that it can satisfy these critical dynamic requirements. The flame combustion can be initiated by electrical ignition. When the ignition is active, rapidly varying fuel gas streams are transformed into a flame. The emissions during this heterogeneously supported combustion are higher than in a case of purely catalytic operation, but nevertheless can be kept within reasonable limits.
In other words, the activation of the ignition is adapted to the load cycle in each case. A well-thought-through regulation principle is necessary for this in order to assure that the necessary heat balance is maintained for the endothermic zones. This means the regulation of the material flows of fuel gas and air, the distribution of the material flows over the catalytic surface, the maintenance of a corresponding surface temperature and the switching on of the ignition when required. The catalytic converter can be dimensioned in such a way that the heterogeneously supported operation with flame combustion is automatically adjusted during dynamic operation, while in the case of steady state operation the purely catalytic operating mode takes place. The balancing out of the heat by radiation, as already stated above, is fast and efficient. It is considered to be a major advantage that the burner arrangement according to the invention is capable of satisfying the dynamic conditions during mobile use within a compact fuel preparation system.
The burner arrangement according to the invention is therefore suitable as a decentralized heat source inside a compact fuel preparation system also for mobile use and offers, taken together, the following advantages:
small volume and low weight,
minimized heat loses due to the sequential arrangement of heat sources and heat sinks,
efficient and rapid coupling in of heat by radiation,
possibility of fast load change cycles,
minimization of the emission of pollutants by catalytic combustion;
selective management of heat and material balances.